The present invention relates to a synchronous separating circuit, and more particularly to a synchronous separating circuit separating or extracting a synchronous signal from a complex image signal for a TV receiver, a video system or the like.
A synchronous separating circuit functions separation of a synchronous signal from a complex image signal constituting a series of signals each including an analog image signal and a synchronous pulse. The waveform and the potential level of an individual image signal depends on color, brightness or chroma of the image to be transmitted.
The synchronous signal separated by a synchronous separating circuit is utilized, for example, in a TV receiver to determine a timing for displaying the image signals transmitted, i.e. it determines horizontal locations at which series of individual image signals are to be displayed on a TV screen. Generally, a synchronous separating circuit is required to output a signal in which the pulse width or period of the output synchronous signal does not change with time.
FIG. 1 shows a conventional synchronous separating circuit for a black-and-white liquid crystal television, in which the separating circuit is constituted by CMOS. The synchronous separating circuit comprises a comparator 53, Pch and Nch MOS transistors 56 and 59, a capacitor 52 and resistors 57 and 58. The input terminal 51 of the synchronous separating circuit is connected to one of the terminals of the capacitor 52, the other terminal of which is connected to a connection point of one of the terminals of the resistor 57 with one of the terminals of the resistor 58 and to the non-inverting input (+) of the comparator 53.
The inverting input (-) of the comparator 53 connected to an electric source 61 of a bias voltage V.sub.B, and the output of the comparator 53 is connected to the output terminal 54 of the synchronous separating circuit and to the gates of the Pch MOS transistor 56 and the Nch MOS transistor 59. The source of the Pch MOS transistor 56 is connected to a higher potential electric source 55 of a constant voltage V.sub.DD, and the drain of the Pch MOS transistor 56 and the other terminal of the resistor 57 are connected to each other. The source of the Nch MOS transistor 59 is connected to a ground 60. The drain of the Nch MOS transistor 59 and the other terminal of the resistor 58 are connected to each other.
Waveforms of a complex image signal to be inputted to the synchronous separating circuit is shown in FIGS. 2A and 2B in somewhat a simplified manner. The waveform of FIG. 2A shows the complex image signal when the image signal included in the complex image signal represents white, while the waveform of FIG. 2B shows the complex image signal when the image signal represents black. As shown in these drawings, the complex image signal has a period t.sub.1 called a synchronous period during which the potential of the signal falls to the minimum and a period t.sub.2 during which the potential of the complex image signal has different values for representing the image to be transmitted.
The operation of the circuit of FIG. 1 is as follows: A complex image signal is fed to one of the terminals of the capacitor 52 through the input terminal 51. The potential of the non-inverting input of the comparator 53 follows the complex image signal except the direct current potential thereof. When the potential of the non-inverting input (node "a") falls below the bias voltage V.sub.B, the output of the comparator 53 becomes "L", thereby closing the Pch MOS transistor 56 and charging the node "a" during a synchronous period t.sub.1. When the potential of node "a" rises above the bias voltage V.sub.B, the output of the comparator 53 becomes "H", thereby closing the Nch MOS transistor 59 and discharging from the node "a" through the Nch MOS transistor 59 during a period t.sub.2. The output of the comparator 53 is a synchronous signal separated by the synchronous separating circuit. Either of the positive- or negative-going transition of the potential of the synchronous signal is utilized as a timing to display an image signal.
The conventional synchronous separating circuit functions a normal separating operation, when electric charge Q1 stored in the node "a" during the period t.sub.1 and electric charge Q2 discharged from the node "a" during the period t.sub.2 are equal to each other. Namely, a normal synchronous separation is carried out when the following equation holds. ##EQU1## wherein i.sub.c and i.sub.D represent a charging current and a discharging current of the node "a", respectively.
However, in a conventional synchronous separating circuit, electric charge discharged from the node "a" during the period t.sub.2 depends on the conditions where the image signal is white, gray or black, so that there is a drawback in the conventional synchronous separating circuit in which the width of the synchronous pulse of the output synchronous signal changes depending on the conditions.